Cranial perforator

ABSTRACT

A new drill-type cranial perforator is disclosed of the type which comprises a front drill head assembly made up of a leading inner drill and a trailing outer drill, and a rear support and drive assembly adapted to enable both drills so long as the leading inner drill is encountering a resistive surface and to disable both drills when the leading inner drill stops encountering the resistive surface.

FIELD OF THE INVENTION

This invention relates to drilling implements for use as surgicalinstruments, and more particularly to cranial perforators.

BACKGROUND OF THE INVENTION

Cranial perforators are special purpose drills which are used to boreholes through the skull during cranial surgery. Such holes may be neededto vent fluids from the region surrounding the brain, to provide smallpassageways to the brain for the insertion and removal of instruments,or to position a cranial saw for subsequent use in removing a largerpiece of the skull.

Regardless of the end use of the hole being made, it is critical thatthe cranial perforator stop its boring action before it encounters--andthereby damages--the delicate dura tissue surrounding the brain, or thebrain itself. To this end, cranial perforators have traditionallyutilized a special "safety construction" designed to permit forwardpenetration by the perforator only so long as the perforator's leadingtip is encountering hard bone, and to halt forward penetration by theperforator as soon as the perforator's leading tip passes through thehard bone and before it encounters the soft tissue beneath the bone.More particularly, cranial perforators have traditionally comprised adrill head assembly having a pair of drills disposed in concentricrelation to one another, with the inner drill leading the outer drillslightly so that a bore-counterbore opening is formed as the perforatorpenetrates into the skull. The two drills are coupled to a rear supportand drive assembly via a special clutch arrangement such that bothdrills are enabled so long as the leading inner drill is encountering aresistive surface (i.e., bone) and both drills are disabled as soon asthe inner drill stops encountering the resistive surface (i.e., as soonas it passes through the bone). Inasmuch as the leading inner drill andtrailing outer drill are adapted to cut in a bore-counterborearrangement, the shoulder of bone formed at the intersection of thebore-counterbore opening impedes further progress of the perforatortoward the brain once the leading inner drill is disabled. As a result,the surgeon using the cranial perforator can concentrate entirely on theplacement of the cranial hole and need not fear that the perforator willpenetrate too far into the head so as to damage the delicate dura tissueor the brain itself.

Cranial perforators using the aforementioned "safety construction" arebelieved to have been marketed by a number of different entities, amongthem Codman & Shurtleff of Randolph, Mass. (see U.S. Pat. No. 2842131),Aesculap of Tuttlengen, West Germany, and a medical instrumentssubsidiary of the 3M Corporation of Minneapolis, Minn. Such cranialperforators have included both reusable and disposable models.

Unfortunately, the cranial perforators developed prior to this inventionare believed to suffer from one or more serious deficiencies. Forexample, some of the prior art cranial perforators are believed to beunreliable with regard to the operation of their special "safetyconstruction". Such unreliability is intolerable since a failure of the"safety construction" to operate as intended can have catastrophiceffects. At least some prior drill-type cranial perforators of the typedescribed above have been prone to failure or to unreliable operation oftheir "safety construction" under non-axial loading. Because the priorsafety construction usually involves a pair of diametrically-opposedmembers coupling the drill head assembly to the rear support and driveassembly, non-axial loading of the drill head assembly can give rise toa chattering action of sufficient magnitude to prematurely terminate theuseful life of the safety construction of the perforator or to cause thesafety construction to operate unreliably, e.g. possibly byintermittently disengaging the drill head assembly from the rear supportand drive assembly. Other problems with prior art devices involvedifficulty of manufacturing component parts with necessary precision, sothat both quality and cost tend to suffer. Furthermore, it appears thatvirtually all or many prior art cranial perforators have provendifficult to center when starting a bore. This is particularly true whenthe bore site is well lubricated by blood or other fluids, so that theperforator has a tendency to skate or slip along the outside of theskull. This can result in unnecessary added injury to the patient. Also,at least some prior art cranial perforators tend to render unusable thecranial material removed from the bore site, with the result that theholes made by such perforators may have to be refilled with foreignmaterials at the close of surgery rather than with the patient's ownbone matter. Also, most, if not all, prior art cranial perforators haverequired relatively high speed drilling (i.e., drilling at speeds ofaround 800-1000 RPM), which tends to be less desirable than low-speeddrilling (i.e., drilling at speeds of around 100 RPM) for a variety ofreasons.

In addition to the foregoing, in some prior art reusable cranialperforators, the rear support and drive assembly and/or the clutch meansare inadequately designed to withstand repetitive non-axial loading,with the result that they fail after an indefinite period of use. Also,many prior art reusable cranial perforators are designed so that theymust be disassembled into numerous pieces to effect proper cleaning,with the result that disassembly and subsequent reassembly tend to befairly complicated and time-consuming. Moreover, with many prior artdesigns, if reassembly is improperly effected, the perforator's special"safety construction" may be rendered totally inoperative, in which casethe perforator's forward penetration will not be halted automatically assoon as its leading tip passes through the skull, and damage to delicatetissues can ensue unless the surgeon is skillful and observant enough toprevent penetration into the dura.

In addition to the foregoing, disposable cranial perforators heretoforeknown have generally lacked safeguards to ensure that the perforatorcannot be reused.

OBJECTS OF THE PRESENT INVENTION

Accordingly, one of the objects of the present invention is to provide anovel and improved drill-type cranial perforator having a reliable"safety construction" designed to permit forward penetration by theperforator only so long as the perforator's leading tip is encounteringhard bone, and to halt forward penetration by the perforator as soon asthe perforator's leading tip passes through the hard bone and before itencounters the soft tissue beneath the bone.

Another object is to provide a cranial perforator which has improvedmeans for coupling the front drill head assembly to the rear support anddrive assembly, so as to effectively assure automatic de-clutching ofthe drill head assembly from the support and drive assembly as the tipof the perforator penetrates the far side of the bone, even when theperforator is manipulated under non-axial loads.

Another object is to provide a cranial perforator which is relativelyeasy to manufacture with high precision tolerances.

Another object is to provide a cranial perforator which includes novelcentering means to assist in centering the perforator when starting abore, so as to minimize the tendency of the perforator to skate over theouter surface of the the skull.

Yet another object is to provide a cranial perforator which is adaptedto remove bone material from a cranial opening in a form best suited forsubsequent repacking in the opening when the opening is closed at theconclusion of surgery.

Still another object is to provide a cranial perforator which can beused with relatively slow speed drills (i.e., drills operating at speedsof around 100 RPM), as well as with relatively high speed drills (i.e.,drills operating at speeds of around 800-1000 RPM).

A further object is to provide a reusable cranial perforator betteradapted to withstand repetitive non-axial loading, so that theperforator will not tend to fail after an indefinite period of use.

Still another object is to provide a reusable cranial perforator whichhas a long operational lifetime, is easy to disassemble and toreassemble so as to facilitate cleaning, and which disassembles intorelatively few pieces so as to facilitate dissassembly and reassembly.

Another object is to provide a reusable cranial perforator which will berendered totally inoperative if it should be reassembled incorrectly.

A further object is to provide a disposable cranial perforator whichcannot be disassembled without rendering the perforator totallyinoperative.

Still another object is to provide a disposable cranial perforator whichwill indicate when it has been resterilized in an attempt to use it morethan once.

And another object is to provide a novel drill which can be used to boreholes in various kinds of bone matter, e.g. cranial bone, breast bone,etc.

SUMMARY OF THE INVENTION

A new drill-type cranial perforator is disclosed of the type whichcomprises a front drill head assembly made up of a leading inner drilland a trailing outer drill, and a rear support and drive assemblyadapted to enable both drills so long as the leading inner drill isencountering a surface offering a predetermined loading and to disableboth drills when the leading inner drill stops encountering thepredetermined loading. The new perforator is characterized by one ormore of the following novel features: (1) a unique clutch arrangementcoupling the front drill head assembly to the rear support and driveassembly and arranged so as to avoid decoupling and minimize stressesimposed by non-axially directed loading; (2) a novel centering means onthe leading drill member to prevent "skating"; (3) a unique support anddrive assembly designed to retard failure caused by repetitive non-axialloading; and (4) a unique support and drive assembly that preventsdisassembly and reuse of the perforators. Other features and advantagesare rendered obvious from the following detailed description.

BRIEF DESCRIPTION OF THE DRAWINGS

These and other objects and features of the present invention are morefully disclosed or rendered obvious by the following detaileddescription of the preferred embodiment of the invention, which is to beconsidered together with the accompanying drawings wherein like numbersrefer to like parts, and further wherein:

FIG. 1 is a side elevation of a reusable cranial perforator whichcomprises the preferred embodiment of the present invention;

FIG. 2 is a side elevation of the drill head assembly of the samereusable cranial perforator rotated 60 degrees from the position shownin FIG. 1;

FIG. 3 is a front elevation of the same reusable cranial perforator,taken from the viewpoint represented by line 3--3 in FIG. 1;

FIG. 4 is a side elevation, partially in section, of the same reusablecranial perforator, with the perforator's support and drive assemblyrotated 90 degrees from the position shown in FIG. 1;

FIG. 5 is a cross-section of the same reusable cranial perforator, takenalong line 5--5 of FIG. 4;

FIG. 6 is an exploded perspective view of the drill head assembly of thesame reusuable cranial perforator;

FIG. 7 is a rear elevation of the outer drill of the same reusuablecranial perforator, taken from the viewpoint represented by line 7--7 inFIG. 6;

FIG. 8 is a rear elevation of the inner drill of the same reusuablecranial perforator, taken from the viewpoint represented by line 8--8 inFIG. 6;

FIG. 9 is a side elevation, partially in section, of a disposablecranial perforator which comprises an alternative embodiment of thepresent invention;

FIG. 10 is an exploded perspective view of the drill head assembly ofthe same disposable cranial perforator;

FIG. 11 is a rear elevation of the outer drill of the same disposablecranial perforator, taken from the viewpoint represented by line 11--11in FIG. 10;

FIG. 12 is a rear elevation of the inner drill of the same disposablecranial perforator, taken from the viewpoint represented by line 12--12in FIG. 10; and

FIG. 13 is an enlarged fragmentary showing an alternative embodiment ofthe inner drill's lugs.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT

Looking first at FIG. 1, there is shown a reusable cranial perforatorconstituting a preferred embodiment of the present invention. As seen inFIG. 1, the reusable cranial perforator generally comprises a frontdrill head assembly 100 and a rear support and drive assembly 200. Frontdrill head assembly 100 comprises an inner drill or drill member 102 andan outer drill or drill member 104.

Inner drill 102 is shown in FIGS. 1-4, 6 and 8. Drill 102 is generallycylindrical in nature and comprises a cylindrical midsection 106 (FIG.6). The front end of drill 102 is dissected by a plurality of inclinedintersecting surfaces so as to define three prismatic flutes or bladesidentified generally at 108. More particularly, the three flutes 108comprise a trio of first inclined surfaces 110, a trio of secondinclined surfaces 112, and a trio of third inclined surfaces 114, plus atrio of end surfaces 116, with each of the latter being intersected bysurfaces 110 and 112 of one flute and surface 114 of another flute.Flutes 108 are disposed 120 degrees apart from one another. Accordingly,each of the surfaces 110, 112 and 114 of each flute is displaced 120degrees from the corresponding surface of the other two flutes. Onaccount of the relative dispositions of the inclined surfaces 110, 112,and 114, each of the flutes 108 includes a front end notch 117, and theinner drill terminates in a pyramidal end projection 118 which extendsoutward beyond the front end surfaces 116 of flutes 108 (FIGS. 2 and 6).The planes of surfaces 114 are eccentric to the lead point of pyramidalend projection 118, and end surfaces 116 are pitched at a 61/2 degreeangle in the circumferential (i.e., non-radial) direction. The leadingedges of surfaces 116 constitute front cutting edges. The outer edges ofsurfaces 114 also constitute cutting edges.

The rear end of cylindrical midsection 106 terminates in an end surfaceor wall 124 (FIGS. 4, 6 and 8). A trio of lugs or keys or fingers 126extend rearward from end surface 124. Rearwardly projecting lugs 126 areformed integral with cylindrical midsection 106 and are disposed 120degrees apart from one another. Each of the lugs 126 is shaped so thatit has a first side surface 128 which extends parallel to the centeraxis of drill member 102 and perpendicular to end surface 124, an endsurface 130 which extends substantially parallel to end surface 124, asecond side surface 132 which extends substantially perpendicular to endsurface 124 (and end surface 130), and a third side surface 134 whichextends at an inclined angle (i.e., non-perpendicular) to end surface124. A small groove 136 is formed at the intersection of each inclinedside surface 134 and end surface 124.

Inner drill 102 also includes an axial bore 137 which begins at rear endsurface 124 of cylindrical midsection 106 and terminates in the middleof midsection 106, and a somewhat shallower threaded counterbore 138which begins at rear end surface 124 of cylindrical midsection 106 andterminates at a shoulder 139 in the middle of midsection 106 (FIGS. 4and 8).

Outer drill 104 is shown in FIGS. 1-4, 6 and 7. Outer drill 104 isgenerally cylindrical in nature, and is cut away in a selected manner soas to form a series of flutes or blades at its front end. Moreparticularly, outer drill 104 comprises a substantially cylindrical rearsection 142 which is joined to a generally cylindrical front section 144by a substantially frustoconical section 146 (FIGS. 2, 3 and 6). Rearsection 142 terminates in a rear surface 148 (FIGS. 2, 4, 6 and 7).Outer drill 104 includes an axial bore 152 (FIGS. 4 and 7), and threeinwardly extending lips or dogs 147 having forward surfaces 154 (FIG. 4)and curved inside surfaces 150 which are arcs of a circle concentric tothe axis of the outer drill. Outer drill 104 also includes a trio ofslots 156 extending between lips 147. Slots 156 are spaced 120 degreesapart from one another. Each of the slots 156 forms a shoulder 158. Eachof the lips 147 has side wall surfaces 159A and 159B. Lips 147 arebevelled away at their forward sides so that surfaces 159C extendbetween side wall surfaces 159B and forward surfaces 154. Surfaces 159Care planar in nature and extend at a 45 degree angle to side wallsurfaces 159B and at a 45 degree angle to forward surfaces 154, forreasons which will hereinafter be made clear.

Referring next to FIGS. 1, 2, 3, 4, and 6, the outer drill's generallycylindrical front section 144 is dissected by a plurality of inclinedintersecting surfaces so as to define three flutes or blades identifiedgenerally at 160. More particularly, the three flutes comprise a trio offirst inclined surfaces 162, a trio of second inclined surfaces 164, anda trio of third inclined surfaces 166 (FIGS. 2 and 3). Flutes 160 aredisposed 120 degrees apart from one another, and each terminates in afront end surface 168 (FIGS. 3 and 6). Front end surfaces 168 arepitched at a 3 degree angle in the circumferential (i.e., non-radial)direction. The leading edges of surfaces 168 are front cutting edges,while the outer edges of surfaces 166 constitute side cutting edges.

Inner drill 102 and outer drill 104 are assembled concentrically oneinside the other so as to form the complete drill head assembly 100.More particularly, inner drill 102 and outer drill 104 are positioned inthe manner shown in FIG. 6, i.e., so that the inner drill's flutes 108are aligned with the outer drill's flutes 160, and so that the innerdrill's lugs 126 are aligned with the outer drill's slots 156. Then thetwo drill members are brought together, so that the inner drill slipsinside and makes a close sliding fit with the outer drill, with theinner drill's end wall 124 coming to rest against the forward surfaces154 of lips 147 (FIG. 4). The various parts of the inner and outerdrills are sized and shaped so that when the drill head assembly is puttogether with the inner drill's end surface 124 engaging the outerdrill's surfaces 154, and the lugs 126 are located in slots 156, theouter drill's front end cutting surfaces 168 will be aligned with andbehind the inner drill's front end surfaces 116, the outer drill's firstinclined surfaces 162 will form a rearward extension of the innerdrill's first inclined surfaces 110, the outer drill's second inclinedsurfaces 164 will form a rearward extension of the inner drill's secondinclined surfaces 112, and the outer drill's third inclined surfaces 166will form a rearward extension of the inner drill's third inclinedsurfaces 114 (FIGS. 2, 3 and 4). In addition, the inner drill's lugs 126are sized so that when the inner drill's end wall 124 engages the outerdrill's surfaces 154, the lugs 126 extend out through the outer drill'sslots 156, with the lugs' first side surfaces 128 residing adjacent andparallel to side surfaces 159A of lips 147, and the lugs' inclined sidesurfaces 134 residing adjacent and parallel to bevelled surfaces 159C oflips 147. In addition, the inner drill's lugs 126 are sized so that theyextend out beyond the outer drill's rear surface 148 when the innerdrill's end wall 124 engages surfaces 154 of outer drill 104 (FIGS. 1and 4).

It is to be appreciated that the foregoing assembly can be achieved onlyif inner drill 102 and outer drill 104 are properly aligned with oneanother (i.e., so that the inner drill's flutes 108 are aligned with theouter drill's flutes 160, and so that the inner drill's keys 126 arealigned with the outer drill's slots 156) prior to moving the two drillsinto engagement. On account of the size and shape of the inner drill'slugs 126 and the size and shape of outer drill 104, if the lugs 126 arenot properly aligned with the outer drill's slots 156 when the two drillmembers are moved together, the end surfaces 130 of the inner drill'skeys 126 will encounter the forward surfaces 154 of lips 147 and therebyprevent the inner and outer drills from achieving the position shown inFIG. 4.

Rear support and drive assembly 200 is shown in FIGS. 1, 4 and 5.Assembly 200 comprises a cylindrical outer sleeve 202. Sleeve 202includes an axial bore 204, a first axial counterbore 206, a secondaxial counterbore 208, and a third axial counterbore 210. Axial bore 204begins at the sleeve's front end surface 212 and extends rearward tocounterbore 206. A shoulder 214 is formed at the intersection of bore204 and counterbore 206. Counterbore 206 in turn extends rearward tocounterbore 208. A shoulder 216 is formed at the intersection ofcounterbore 206 and counterbore 208. Counterbore 208 extends rearward tocounterbore 210. A shoulder 218 is formed at the intersection ofcounterbore 208 and counterbore 210. Counterbore 210 intersects thesleeve's rear end surface 220. Sleeve 202 also includes a trio ofkey-receiving recesses 222 formed in the sleeve's front end surface 212(FIGS. 4 and 5). Recesses 222 are spaced 120 degrees apart from oneanother and have bottom surfaces 224. Each of the recesses 222 isdefined by side wall surfaces 225A and 225B. A radial bore 226 extendsthrough the side wall of sleeve 202.

An annular seal 228 (FIG. 4) is disposed within counterbore 206concentric with the axis of sleeve 202. Seal 228 has a C-shapedcross-section and is formed out of a resilient material, e.g. a softrubber. An expander element, e.g. a resilient 0-ring 230, is positionedinside the seal to keep it radially extended for the purpose hereinafterdescribed.

An annular spacer element 234 is disposed within counterbore 208. Spacerelement 234 rests against shoulder 216 and is sized so that its innersurface lies flush with the inner surface of sleeve 202.

Also disposed within counterbore 208 are three annular bearing members238. Bearing members 238 are sized so that their innermost surfaces areflush with the surface of sleeve 202 which defines bore 204, and alsowith the innermost surface of spacer element 234.

A coupling or connecting pin 242 is slidably disposed within sleeve 202and annular members 228, 234 and 238. Pin 242 is formed with acylindrical midsection 243, a threaded cylindrical reduced diameterfront section 244, and a rear flange 245 having an enlarged diameterrelative to cylindrical midsection 243. A shoulder 246 is formed at theintersection of threaded cylindrical front section 244 and cylindricalmidsection 243, and rear flange 245 terminates in an end surface 248.Connecting pin 242 is sized so that its cylindrical midsection 243 makesa close sliding fit in bore 204 of sleeve 202, and also with theinnermost surfaces of seal 228 and spacer element 234 and bearings 238.As a result, connecting pin 242 is free to move independently of sleeve202. At the same time, on account of the fact that the inside wall ofseal 228 is urged by expander element 230 to assume a position slightlyfurther inward than the surface of sleeve 202 which defines bore 204,resilient seal 228 engages and makes a good seal with the outer surfaceof the connecting pin's cylindrical midsection 243. This engagement issufficient to prevent liquid or solid substances from passing betweenseal 228 and the connecting pin, but it is not sufficient tosignificantly inhibit the movement of connecting pin 242 relative tosleeve 202. Connecting pin 242 also includes an axial bore 249 whichbegins at its rear surface 248 and extends into the pin's cylindricalmidsection, and a radial bore 250 which begins at the pin's outsidesurface and extends into midsection 243. Radial bore 250 is positionedso that it will be aligned with the sleeve's bore 226 when theconnecting pin's rear flange 245 is in engagement with the rearmostbearing 238, and radial bore 250 has a diameter identical to thediameter of radial bore 226.

Means are provided to urge connecting pin 242 forward so that the pin'srear flange 245 normally engages the rearmost bearing 238, in the mannershown in FIG. 4. More particularly, the rear support and drive unit 200includes a drive adapter 252 which closes off the rear of sleeve 202.Adapter 252 has a stepped-down exterior configuration at its rear endwhich is adapted to be received by a Hudson chuck, as will hereinafterbe described in further detail. Adapter 252 includes an axial bore 254,an axial counterbore 256, a shoulder 258 formed at the intersection ofbore 254 and counterbore 256, and a peripheral flange 259. Adapter 252is press fitted into the sleeve's counterbore 208, with its peripheralflange 259 making a close fit in the sleeve's counterbore 210. The innerend of adapter 252 engages the rearmost bearing 238 and therebycaptivates the bearings in sleeve 202. A thrust bearing unit 260 havinga circular peripheral flange 262 is disposed in the end cap's bore 254and counterbore 256, in the manner shown in FIG. 4. Bearing unit 260includes an axial through-hole 264 which accommodates a mandrel orspring pin 268 which serves as an anchor for a compression spring 266.The latter extends into bore 249 of pin 242. This construction sufficesto keep the connecting pin's rear flange 245 biased against the rearmostbearing member 238, without significantly impeding the rotation ofconnecting pin 242 relative to sleeve 202. At the same time, pin 242 iscapable of axial motion relative to sleeve 202 to the extent permittedby the gap normally residing between the pin's flange 245 and thrustbearing 260.

It will be appreciated that rear support and drive assembly 200essentially forms a self-contained unit wherein connecting pin 242projects its threaded front end 244 outward from the front end of sleeve202 and is yieldably biased into that position, and further wherein theconnecting pin is capable of rotation relative to sleeve 202.

The front drill head assembly 100 is united with the rear support anddrive assembly 200 by screwing the connecting pin's threaded cylindricalfront section 244 into the inner drill's threaded counterbore 138, sothat the connecting pin's shoulder 246 engages the inner drill's endsurface 124.

The various parts of the cranial perforator are sized so that the drillhead assembly can only be screwed onto connecting pin 242 when the innerdrill's lugs 126 extend through the outer drill's slots 156, for reasonswhich will hereinafter be described in detail. In addition, the variousparts of the cranial perforator are sized so that when the drill headassembly and the support and drive assembly are so united, and theconnecting pin's rear flange 245 is in engagement with the rearmostbearing unit 238, the inner drill's lugs 126 will terminate short of thesleeve's front end surface 212 (FIG. 4). At such time, the lugs areincapable of being locked to sleeve 202 so that sleeve 202 cannot drivethe drill head assembly. At the same time, however, the various parts ofthe cranial perforator are sized so that when the front drill headassembly and the rear support and drive assembly are united in theforegoing manner, and the inner drill 102 is thereafter forcedrearwardly relative to sleeve 202 against the action of spring 266, thelugs 126 can extend into the key-receiving recesses 222 before theconnecting pin's rear surface 248 contacts bearing unit 260, whereby thelugs can lock the drill head assembly to the sleeve so as to cause thetwo to rotate together.

Operation of the cranial perforator will now be described.

The assembled cranial perforator is prepared for use by fitting theperforator's adaptor 252 into a Hudson chuck which is disposed on theend of the drive shaft of a suitable driver. Subsequent rotation of thedrive shaft in a counterclockwise direction (as viewed in FIG. 3) willcause the rear support and drive unit 200 to rotate in the samecounterclockwise direction. On account of some residual friction betweenconnecting pin 242 and the remainder of the rear support and drive unit200, the front drill head assembly 100 will generally tend to rotatewith rear support and drive unit 200 so long as the front drill headassembly is not encumbered by any braking action. However, if anybraking action whatsoever is applied to inner drill 102 while the rearsupport and drive unit 200 is rotating, without the inner drill beingsubjected to a rearward force sufficient to overcome the force of spring266, the perforator's aforementioned construction will allow front drillhead assembly 100 to stop rotating even while rear support and driveunit 200 continues to rotate. Similarly, if any braking action isapplied to outer drill 104 while the rear support and drive unit 200 isrotating and while the inner drill is not subjected to a rearward forcesufficient to overcome the force of spring 266, the non-rotating outerdrill will be cammed backward by virtue of the engagement of theinclined surfaces 134 of lugs 126 with surfaces 159C of lips 147, untilthe outer drill's rear end surface 148 contacts the sleeve's front endsurface 212, and while so positioned the outer drill will be in slidingengagement with the rotating sleeve. As soon as the outer drill'ssurfaces 159B contact side surfaces 132 of the inner drill's lugs 126,rotation of the inner drill will also cease, the continued rotation ofthe rear support and drive unit 200 notwithstanding.

Now when the cranial perforator is to be used to drill a hole in askull, the powered drive unit (not shown) drives the cranial perforatorin a counterclockwise direction. The cranial perforator is brought downso that its pyramidal front projection 118 contacts the skull preciselywhere the cranial hole is to be made. As the sharp pyramidal projection118 keeps the cranial perforator centered, the perforator is presseddown against the skull so that inner drill 102 and connecting pin 242are forced backwards against the pressure of spring 166. This actionallows the inner drill's lugs 126 to enter recesses 222 of the rotatingsleeve 202, so that the surfaces 128 of lugs 126 are engaged by thesleeve's surfaces 225B, with the result that rotation of the sleeve isimparted-to the inner drill. As the inner drill rotates, its pyramidalprojection 118 and its flutes 108 bore into the skull. At the same time,the outer drill's surfaces 159C are engaged by the rotating lugs'surfaces 134, causing the outer drill to rotate in unison with the innerdrill. As the perforator cuts its way into the skull, the leading innerdrill's flutes 108 cut a bore, and the trailing outer drill's flutes 260cut a counterbore, so that a bore-counterbore opening is formed in theskull. Because front end surfaces 168 are cut at a flatter angle thanfront end surfaces 116, the outer drill will tend to encounter greatercutting resistance than the inner drill.

When the leading tip of the inner drill passes through the target bone,so that it no longer meets a resistive surface and is free to slipforward, the camming action of the outer drill's bevelled surfaces 159Cbearing against the inner drill's lug surfaces 134 causes the innerdrill to slip forward relative to the outer drill and the rear supportand drive assembly far enough for lugs 126 to move out of recesses 222and thereby disengage themselves from sleeve 202. With the inner drillno longer coupled to the rear support and drive unit 200, residualfriction with the skull causes the rotation of drills 102 and 104 tocease. Further forward penetration of the cranial perforator is impededat this point, inasmuch as the bore-counterbore made by the cranialperforator has formed a solid shoulder of bone which blocks the frontsurfaces 168 of the now-stationary outer drill. The cranial perforatormay be removed from the cranial opening simply by pulling it backward.

On account of the number and shape of the inner drill's lugs 126 and thenumber and shape of the sleeve's lug-receiving recesses 222, and also onaccount of the manner in which connecting pin 242 is maintained withinsleeve 202, the coupling between the front drill head assembly 100 andthe rear support and drive assembly 200 provides reliable service duringdrilling even when the perforator is subjected to a variety of non-axialloadings. Three lugs 126 are provided so as to prevent wobbling of theinner drill, maintain positive locking of the inner drill to the sleeve202 at all times when the perforator is pressed tightly against bone,and assure that de-clutching of the drill head assembly will occurautomatically whenever the inner drill encounters a drop in resistancefrom the surface which it is drilling. Using two or four lugs forcoupling the inner drill to the sleeve is undesirable since then thereis a tendency for the inner drill to wobble, i.e., to shift laterallyabout a transverse pivot axis under non-axial loading. This wobblingaction is detrimental since it creates a friction buildup between theinner and outer drills which may be great enough to prevent reliablede-clutching of the drill head assembly.

Because of the unique construction of the inner drill's flutes 108 andthe outer drill's flutes 160, drilling can be conducted at relativelylow speeds (i.e., at speeds of around 100 RPM), rather than at therelatively high speeds (i.e., speeds of around 800-1000 RPM) required ofprior art devices in order for them to function properly.

Furthermore, the particular shapes of the inner drill's flutes 108 andthe outer drill's flutes 160 enable removal of bone material from thecranial opening in a form best suited for subsequent repacking in theopening when the opening is closed at the conclusion of surgery.

The cranial perforator described and illustrated above is intended to bereused numerous times before being discarded. At the conclusion of theoperation, the perforator may be easily disassembled for more completecleaning. To accomplish disassembly, the front drill head assembly isrotated so that the attached connecting pin's radial bore 250 is alignedwith the sleeve's radial bore 226. Then a tool is inserted into radialbores 226 and 250 so as to lock the connecting pin against rotationrelative to sleeve 202. Thereafter, the front drill head assembly isunscrewed from the locked connecting pin, and the inner drill isseparated from the outer drill. The three parts (i.e., the inner drill,the outer drill, and the rear support and drive assembly) may then bewashed and sterilized. In this respect it is noted that the rear supportand drive assembly generally requires no further disassembly for propercleaning inasmuch as seal 228 prevents material from making its way intothe rear portion of the support and drive assembly.

By carefully sizing the various parts of the cranial perforator so thatfront drill head assembly 100 cannot be screwed onto connecting pin 242unless the inner drill's lugs 126 extend through the outer drill's slots156, it is assured that the inner and outer drills can never be lockedin position relative to one another so as to defeat the perforator'sspecial "safety construction". Thus, reassembly of the cranialperforator after cleaning is rendered virtually foolproof.

FIGS. 9-12 show a disposable cranial perforator which comprises analternative embodiment of the present invention. This alternativeembodiment comprises a front drill head assembly 300 and a rear supportand drive assembly 400 (FIG. 9). Front drill head assembly 300 comprisesan inner drill 302 and an outer drill 304.

Inner drill 302 is shown in FIGS. 9, 10 and 12. Inner drill 302 issimilar in shape to the inner drill 102 previously described. Moreparticularly, the front end of inner drill 302 is identical to the frontend of inner drill 102, inasmuch as the drill's cylindrical midsection306 is dissected by a plurality of intersecting inclined surfaces so asto form three flutes 308 (FIG. 10). Specifically, the three flutescomprise a trio of first inclined surfaces 310, a trio of secondinclined surfaces 312, and a trio of third inclined surfaces 314, plus atrio of end surfaces 316, with each of the latter being intersected bysurfaces 310 and 312 of one flute and surface 314 of another flute.Flutes 108 are disposed 120 degrees apart from one another. Accordingly,each of the surfaces 310, 312 and 314 of each flute is displaced 120degrees from the corresponding surface of the other two flutes. Onaccount of the relative dispositions of inclined surfaces 310, 312, and314, each of the flutes 308 has a front end notch 317, and the innerdrill terminates in a pyramidal projection 318 which extends outwardbeyond the front end surfaces 316 of flutes 308 (FIGS. 9 and 10). Theplanes of surfaces 314 are eccentric to the lead point of pyramidal endprojection 318, and end surfaces 316 are pitched at a 61/2 degree anglein the circumferential (i.e., non-radial) direction.

Inner drill 302 also comprises a cylindrical rear section 320 which isformed integral with cylindrical midsection 306. Cylindrical rearsection 320 has a slightly smaller diameter than cylindrical midsection306, so that an exterior shoulder 322 is formed at the intersection ofthese two sections (FIGS. 10 and 12). Cylindrical rear section 320terminates in an end wall 324. A trio of lugs or keys 326 extendrearward from end wall 324. Lugs 326 are formed integral withcylindrical rear section 320 and are disposed 120 degrees apart from oneanother. Each of the lugs 326 is shaped so that it has a first sidesurface 328 which extends perpendicularly outward from end surface 324,an end surface 330 which extends substantially parallel to end surface324, and a second side surface 332 which extends at an inclined angle(i.e., non-perpendicular) to end surface 324. A small groove 336 isformed in cylindrical rear section 320 at the intersection of eachinclined side surface 332 and end surface 324.

Inner drill 302 also includes an axial bore 338 which begins at rear endsurface 324 of cylindrical rear section 320 and extends into the middleof cylindrical midsection 306, and a threaded axial counterbore 339which begins at rear end surface 324 of cylindrical rear section 320 andterminates at a shoulder 340 in the middle of midsection 306 (FIGS. 9and 12).

Outer drill 304 is shown in FIGS. 9, 10 and 11. Outer drill 304 issimilar to the outer drill 104 previously described. More particularly,outer drill 304 comprises a substantially cylindrical rear section 342which is formed integral with a generally cylindrical front section 344.Front section 344 has a larger outside diameter than cylindrical rearsection 342, and an exterior shoulder 346 is formed at theirintersection. Cylindrical rear section 342 generally terminates in anend surface 348. An axial bore 349 passes through front section 344 andcylindrical rear section 342. Outer drill 304 also includes three lipsor dogs 351 at its rear end. Lips 351 are formed integral withcylindrical rear section 342 and extend inwardly of rear section 342.Lips or dogs 351 are disposed 120 degrees apart from one another, andare sized and shaped so as to define a trio of radial extending slots353 therebetween. Lips 351 terminate in arcuate inner surfaces 355, sidesurfaces 356A and 356B, and parallel opposite end surfaces 357 and 359.

The generally cylindrical front section 344 of outer drill 304 isdissected by a plurality of intersecting inclined surfaces so as todefine three flutes or blades 360. More particularly, the three flutes360 comprise a trio of first inclined surfaces 362, a trio of secondinclined surfaces 364, and a trio of third inclined surfaces 366. Flutes360 are disposed 120 degrees apart from one another, and each terminatesin a front end surface 368. Front end surfaces 368 are pitched at a 3degree angle in the circumferential (i.e., non-radial) direction.

Inner drill 302 and outer drill 304 are assembled in a concentric mannerso as to form the complete front drill head assembly 300. Moreparticularly, inner drill 302 and outer drill 304 are positioned in themanner shown in FIG. 10, i.e., so that the inner drill's flutes 308 arealigned with the outer drill's flutes 360, and so that the inner drill'slugs 326 are aligned with the outer drill's radial slots 353. Then thetwo drill members are brought together so that the inner drill slipsinside and makes a close sliding fit with the outer drill, with theinner drill's end surface 324 coming to rest against the inner endsurfaces 357 of the outer drill's lips 351 (FIG. 9). The various partsof the inner and outer drills are sized and shaped so that when theinner drill's end surface 324 engages the outer drill's inner endsurfaces 357, and the lugs 326 are located in slots 353 so that theouter drill's front end surfaces 368 are in alignment with but rearwardof the inner drill's front end surfaces 316, the outer drill's inclinedsurfaces 362 will form a rearward extension of the inner drill'sinclined surfaces 310, the outer drill's inclined surfaces 364 will forma rearward extension of the inner drill's inclined surfaces 312, and theouter drill's inclined surfaces 366 will form a rearward extension ofthe inner drill's inclined surfaces 314. In addition, the inner drill'slugs 326 are sized so that when the inner drill's end surface 324engages the outer drill's inner end surfaces 357, and the inner drill'sflutes 308 are aligned with the outer drill's flutes 360, the lugs 326will extend out through the outer drill's radial slots 353, with thefirst side wall surfaces 328 of the lugs extending parallel to andslightly spaced from side surfaces 356A of lips 351. The inner drill'slugs 326 are also sized so that they project out beyond the outer endsurfaces 359 of the outer drill's lips 351 when the inner drill's endsurface 324 engages the outer drill's inner end surfaces 357.

It is to be appreciated that the foregoing assembly can be achieved onlyif inner drill 302 and outer drill 304 are properly aligned with oneanother (i.e., so that the inner drill's lugs 326 are aligned with theouter drill's slots 353) prior to moving the two drills into engagement.More specifically, on account of the size and shape of lugs 326 and thesize and shape of outer drill 304, if the lugs 326 are not properlyaligned with the slots 353 when the two drill members are movedtogether, the end surfaces 330 of lugs 326 will encounter the inner endsurfaces 357 of the outer drill's lips 351 and thereby prohibit theinner and outer drills from achieving the position shown in FIG. 9.

Rear support and drive assembly 400 is shown in FIG. 9. Assembly 400comprises a hollow inner sleeve 401 having a threaded body section 403and a substantially cylindrical collar section 405 at its front end.Collar section 405 has a larger outside diameter than body section 403,and an exterior shoulder 407 is formed at their intersection. Collarsection 405 terminates in a front end surface 409, and body section 403terminates in a rear end surface 411. A trio of key-receiving recesses413 are formed in the sleeve's front end surface 409. Recesses 413 aresimilar in shape to the aforementioned openings 222 formed in theaforementioned rear support and drive assembly 200, and are spaced 120degrees apart from one another. Recesses 413 have bottom surfaces 415.The axial bore in sleeve 401 is identified by numeral 419.

A coupling or connecting pin 421 is slidably disposed within sleeve 401.Connecting pin 421 has a cylindrical midsection 423, a cylindrical frontsection 425 having a reduced diameter relative to cylindrical midsection423, and a rear flange 427 having an enlarged diameter relative tocylindrical midsection 423. A shoulder 429 is formed at the intersectionof cylindrical front section 425 and cylindrical midsection 423.Cylindrical midsection 423 is threaded for a short distance rearward ofshoulder 429. Connecting pin 421 is sized so that its cylindricalmidsection 423 makes a close sliding fit in bore 419 of sleeve 401, inorder that connecting pin 421 will be capable of independent movementrelative to sleeve 401. Connecting pin 421 also includes an axial bore431 extending forward from the rear end surface 433 of rear flange 427.

Means are provided to urge connecting pin 421 forward so that the pin'srear flange 427 normally engages the rear end surface 411 of innersleeve 401, in the manner shown in FIG. 9. More particularly, the rearsupport and drive unit 400 includes a drive adaptor 435 which fits oversleeve 401. Adaptor 435 has a rear section 436 having a stepped-downouter configuration adapted to be received by a Hudson chuck. Adaptor435 includes an axial bore 437, an axial counterbore 439, and a shoulder441 formed at the intersection of bore 437 and counterbore 439.Counterbore 439 is threaded for a distance rearward of shoulder 441 soas to permit the adaptor to be screwed onto sleeve 401, with thethreaded sleeve's collar section 405 fitting in counterbore 439 and thesleeve's shoulder 407 engaging the adaptor's shoulder 441. A compressionspring 443 is located in the connecting pin's bore 431 so as to urge theconnecting pin forward away from the adaptor's interior end surface 445and thus keep flange 427 of the connecting pin biased against the innersleeve's rear end surface 411. This construction suffices to keep theconnecting pin's rear flange 427 biased against the inner sleeve's endsurface 411 without significantly impeding rotation of connecting pin421 relative to sleeve 401. At the same time, pin 421 is capable ofaxial motion relative to sleeve 401 to the extent permitted by the gapnormally extending between flange 427 and end surface 445.

As a consequence of the foregoing construction, it will be appreciatedthat rear support and drive unit 400 essentially forms a self-containedunit wherein connecting pin 421 projects its threaded front end outwardfrom inner sleeve 401 and adaptor 435 and is yieldably biased into thatposition, and further wherein the connecting pin is capable ofindependent rotation relative to sleeve 401 and adaptor 435.

The front drill head assembly 300 is united with the rear support anddrive assembly 400 by screwing connecting pin 421 into the inner drill'sbore 338 and threaded counterbore 339, until the connecting pin'sshoulder 429 seats on the inner drill's shoulder 340. The various partsof the disposable cranial perforator are sized so that when spring 443is holding the connecting pin's rear flange 427 against the rear endsurface 411 of inner sleeve 401, the inner drill's lugs 326 willterminate short of the sleeve's front end surface 409. At the same time,however, the various parts of the cranial perforator are sized so thatwhen the inner drill is forced back towards rear support and driveassembly 400, lugs 326 can extend into the lug-receiving recesses 413formed in sleeve 401 and the rear end surface 359 of outer drill 304 canengage the front end surface 409 of sleeve 401, before the rear endsurface 433 of the connecting pin contacts end surface 445. Adaptor 435is sized so that when the front drill head assembly is united with therear support and drive assembly, the front end of the adaptor willextend over the rear end of the front drill head assembly in the mannershown in FIG. 9.

The adaptor 435 is made to extend over the rear end of the front drillhead assembly for a most important reason; specifically, thisconstruction makes it impossible for an assembled perforator to bedisassembled, with the result that reuse of the perforator iseffectively prohibited so long as sterile conditions are a requisite.Disassembly is prevented inasmuch as the freely rotating connecting pin421 must be held stationary in order for the front drill head assemblyto be detached from the remainder of the perforator, and the connectingpin is rendered inaccessible on account of the fact that inner sleeve401 is also inaccessible and therefore cannot be held still to allow forthe unscrewing of adaptor 435 from sleeve 401.

To further inhibit reuse of the disposable cranial perforator, adaptor435 may be formed of a low temperature thermoplastic so that the adaptorwill destruct in a high-temperature autoclave during a sterilizationprocedure. Adaptor 435 also may carry a gas-sensitive label on itsexterior surface to indicate whether the perforator has been subjectedto a gas sterilization procedure.

Operation of the disposable cranial perforator is substantially the sameas the operation of the reusable cranial perforator, and hence need notbe redescribed.

FIG. 13 shows an alternative embodiment of the inner drill's lugs 126.In this case, a corner of each lug 126 is bevelled so as to provide aplanar surface 129 which is interposed between and extends at aninclined angle (i.e., non-perpendicular) to both side surface 128 andend surface 130. Surfaces 129 serve as cam surfaces. They are engaged bythe sleeve's surfaces 225B (FIG. 5) as the inner drill slips forward atthe conclusion of drilling and such engagement assists forward movementof the inner drill relative to sleeve 202 and thereby promotes a morerapid disengagement of the inner drill's lugs 126 from sleeve 202.

Advantages Of The Invention

Numerous advantages are obtained by using the present invention.

First, the present invention provides a cranial perforator having areliable "safety construction" designed to permit forward penetration bythe perforator only so long as the perforator's leading tip isencountering hard bone, and to halt forward penetration by theperforator as soon as the perforator's leading tip passes through thehard bone and before it encounters the soft tissue beneath the bone.

Second, the present invention provides a cranial perforator having animproved clutch means for coupling the front drill head assembly to therear support and drive assembly, the clutch means being designed so asto assure automatic de-clutching even when the perforator is manipulatedunder non-axial loads.

Third, the present invention provides a cranial perforator whichincludes a uniquely formed centering point to assist in centering theperforator when starting a bore, so as to minimize the tendency of theperforator to skate across the skull.

Fourth, the flutes of the cranial perforator of this invention are welladapted to remove the skull material from the cranial opening in a formbest suited for subsequent repacking in the opening when the opening isclosed at the conclusion of surgery.

Fifth, the invention provides a cranial perforator which can be usedwith relatively low speed drills (i.e., drills operating at speeds ofaround 100 RPM), as well as with relatively high speed drills (i.e.,drills operating at speeds of around 800-1000 RPM).

Sixth, the present invention provides a reusable cranial perforatorbetter adapted to withstand repetitive non-axial loading, so that theperforator will not tend to fail after an indefinite period of use.

Seventh, the present invention provides a reusable cranial perforatorwhich has a long operational lifetime, and which is easy to disassembleand to reassemble so as to facilitate cleaning.

Eighth, the present invention provides a reusable cranial perforatorwhich disassembles into relatively few pieces, so as to facilitatedisassembly and reassembly.

Ninth, the present invention provides a reusable cranial perforatorwhich will be rendered totally inoperative if it is reassembledincorrectly.

Tenth, the present invention provides a disposable cranial perforatorwhich cannot be disassembled without rendering the perforator totallyinoperative.

Eleventh, the present invention provides a disposable cranial perforatorwhich will indicate when it has been resterilized in an attempt to useit more than once.

Finally, the invention has the advantage that it is susceptible ofmodifications other than those already described. Thus, the shapes ofthe cutting flutes may be changed. Still other possible modificationswill be obvious to persons skilled in the art. Whst I claim is:

1. A drilling implement for drilling holes in bone structure comprisinga drill head assembly and a drill head drive assembly;said drill headassembly comprising a hollow outer drill member and an inner drillmember coaxially disposed within said outer drill member; said outerdrill member comprising an outer end having a plurality of cuttingflutes and an inner end having three circumferentially-spaced lips thatextend in a plane at right angles to the longitudinal axis of said outerdrill member, each of said lips having a drive surface extending in aplane that intersects the plane of said lips; said inner drill membercomprising an outer end having a plurality of cutting flutes and aninner end having (1) an end surface, (2) threecircumferentially-extending, mutually-spaced lugs projecting from saidend surface para11el to the center axis of said inner drill member, eachof said lugs having a cam surface extending circumferentially and at anangle to the longitudinal axis of said inner drill member and disposedto engage one of said drive surfaces and thereby drive said outer drillmember when said inner drill member is rotated in a first direction, and(3) a threaded center hole; said drill head drive assembly comprising adrive member, a drill head/drill drive coupling member, a drive adaptor,and biasing means; said drive member comprising an end surface extendingtransversely of its axis and interrupted by three recesses for receivingsaid three lugs, each of said recesses being defined in part by sidesurfaces extending in planes extending parallel to the center axis ofsaid drive member; said coupling member being screwed into said threadedcenter hole so that is and said inner drill member will rotate as a unitwhen said inner drill member is rotated in a said first direction; saiddrive adaptor being connected to said drive member so that said driveadaptor and said drive member will rotate as a unit when said driveadaptor is rotated in said first direction; and said biasing means beingdisposed so as to bias said coupling member and said inner drill memberaxially away from said drive adaptor; said drill head assembly and saiddrill head drive assembly being arranged so that when said drive adaptoris rotated in said first direction, (a) so long as said inner drillmember encounters a predetermined resistance to penetration, said lugswill remain engaged with said side surfaces and said inner drill memberwill cause said outer drill member to rotate with it as a unit with saiddrive adaptor, and (b) when said inner drill member no longer encounterssaid predetermined resistance to penetration, said cam surfaces willcoact with said drive surfaces to force said inner drill member forwardrelative to said outer drill member and said drill head drive assemblysufficiently for said lugs and said side surfaces to be disengaged,whereby said inner and outer drill members will slip relative to saiddrive member as said drive adaptor is rotated in said first direction.2. An improved drilling implement according to claim 1 wherein saidadaptor has a tubular front section which surrounds said drive memberand overlaps said inner end of said outer drill member.
 3. An improveddrilling implement according to claim 1 wherein said inner drill memberhas a plurality of shaped flutes with cutting edges, with each flutecharacterized by a plurality of prismatically disposed surfaces, a frontcutting edge on the forward end of said each flute, and alongitudinally-extending cutting edge formed by the intersection of oneof said prismatically disposed surfaces and the outer surface of saidinner drill member, and further wherein said inner drill membercomprises a three-sided pyramidal center point which projects forwardlyof said front cutting edges, and a notch in each flute separating thesaid front cutting edge of said flute from said pyramidal center point.4. An improved drilling implement according to claim 1 wherein saidinner drill member has a plurality of shaped flutes, with each flutecharacterized by a plurality of prismatically disposed surfaces, a frontcutting edge on the forward end of said each flute, and a longitudinalcutting edge formed by the intersection of one of said prismaticallydisposed surfaces and the outer surface of said inner drill member, andfurther wherein said one prismatically disposed surface is formed sothat it is eccentric to the center axis of said inner drill member. 5.An improved drilling implement according to claim 4 wherein said innerdrill member has front end surfaces terminating in said front cuttingedges, and said outer drill member has matching flutes each having afront end surface terminating in a front cutting edge, and furtherwherein the front end surfaces of said inner and outer drill members aredisposed at different angles to the longitudinal axis of said drillingimplement.
 6. An improved drilling implement for drilling holes in bonestructure comprising:a drill head assembly and a drill head driveassembly coupled to said drill head assembly so as to selectively causesaid drill head assembly to be drivingly engaged by or disengaged fromsaid drill head drive assembly; said drill head assembly comprising aninner drill member, an outer drill member, and means for coupling saidinner drill member to said drill head drive assembly; said drive headdrive assembly comprising a drive member, a coupling member, a driveadaptor, and a biasing means; said inner drill member having a first endwith shaped drilling flutes terminating in cutting edges, and a secondend with a plurality of circumferentially spaced lugs projecting fromsaid second end parallel to the axis of said inner drill member; saidlugs each having a cam surface extending in a plane that is inclinedrelative to the longitudinal axis of said inner drill member; said outerdrill member being hollow and rotatably surrounding said inner drillmember, said outer drill member having a first end with shaped drillingflutes terminating in cutting edges, and a second end with a pluralityof dogs and spaces between said dogs sized to permit said lugs toproject between said dogs; said dogs each having a bevelled surface inposition to engage one of said cam surfaces and urge said inner drillmember axially forward relative to said outer drill member and saiddrive member when said inner drill member is being rotated in a selecteddirection; said drive member being hollow and having a limit surfaceextending transversely of the axis of rotation of said drillingimplement, and a plurality of recesses in said limit surface foraccommodating said lugs; said coupling member extending through saiddrive member and being secured to said inner drill member, said couplingmember being rotatable relative to said drive member and being capableof axial movement between a first limit position in which said lugs aredisposed in said recesses and a second limit position in which said lugsare clear of said recesses; said drive adaptor being secured to saiddrive member so as to cause said drive member to rotate with said driveadaptor; and said biasing means being disposed between said driveadaptor and said coupling member so as to urge said coupling member tosaid second limit position, whereby (a) said drill head assembly will befree to rotate relative to said drive assembly if said inner drillmember is not subjected to an axially-applied force directing it towardsaid drive adaptor and (b) said drill head assembly will be locked tosaid drive assembly for rotation therewith when said inner drill memberis forced toward said drive adaptor far enough to place said couplingmember in said first limit position.
 7. An improved drilling implementaccording claim 6 wherein said inner and outer drill members each hasthree flutes.
 8. An improved drilling implement according to claim 6wherein said inner drill member is characterized by a plurality ofshaped drilling flutes with cutting edges, each of said flutes having aplurality of prismatic surfaces, a front cutting edge on the forward endof each of said shaped drilling flutes, a pyramidal center point whichprojects forwardly of said front cutting edges, and a notch in each ofsaid shaped drilling flutes separating said front cutting edges fromsaid pyramidal center point.
 9. An improved drilling implement accordingto claim 6 wherein said adaptor member is coupled to said drive memberby a screw thread connection.
 10. An improved drilling implementaccording to claim 6 wherein said lugs are formed with first surfacesextending parallel to the longitudinal axis of said drilling head andsecond surfaces extending at an acute angle to said axis, and said limitsurface has selected edge surfaces defining the opposite sides of saidrecesses which extend parallel to said longitudinal axis, whereby whensaid first surfaces are engaged with said selected edge surfaces, saidinner drill member will be driven by said drive member as said drivemember is rotated in said first direction.
 11. An improved drillingimplement according to claim 6 wherein each of said lugs is bevelled onone end corner.
 12. An improved drilling implement according to claim 6wherein said drive member and said coupling member are separated andsupported by at least one roller-type bearing member.
 13. An improveddrilling implement according to claim 12 wherein said inner drill memberhas three lugs and said drive member has three recesses for receivingsaid three lugs.
 14. An improved drilling implement for drilling holesin bone structures comprising:a drill head assembly and a drill headdrive assembly coupled to said drill head assembly so as to selectivelycause said drill head assembly to be drivingly engaged by or disengagedfrom said drill head drive assembly; said drill head assembly comprisingan inner drill member, an outer drill member, and means for couplingsaid inner drill member to said drill head drive assembly; said drillhead drive assembly comprising a drive member, a coupling member, adrive adaptor, a biasing means, and bearing means; said inner drillmember having a first end with shaped drilling flutes terminating incutting edges, and a second end with circumferentially spaced lugsprojecting from said second end parallel to the axis of said inner drillmember; said lugs each having a cam surface extending in a plane that isinclined relative to the longitudinal axis of said inner drill member;said outer drill member hollow and rotatably surrounding said innerdrill member, said outer drill member having a first end with shapeddrilling flutes terminating in cutting edges, and a second end with dogsand spaces between said dogs sized to permit said lugs to projectbetween said dogs; said dogs each having a surface in position to engageone of said cam surfaces and urge said inner drill member axiallyforward relative to said outer drill member and said drive member whensaid inner drill member is being rotated in a selected direction; saiddrive member being hollow and having a limit surface extendingtransversely of the axis of rotation of said drilling implement, andrecesses in said limit surface for accommodating said lugs; saidcoupling member extending through said drive member and being secured tosaid inner drill member, said coupling member being rotatable relativeto said drive member and being capable of axial movement between a firstlimit position in which said lugs are disposed in said recesses and asecond limit position in which said lugs are clear of said recesses;said drive adaptor being secured to said drive member so as to causesaid drive member to rotate with said drive adaptor; said biasing meansbeing disposed between said drive adaptor and said coupling member so asto urge said coupling member to said second limit position, whereby (a)said drill head assembly will be free to rotate relative to said driveassembly if said inner drill member is not subjected to anaxially-applied force directing it toward said drive adaptor and (b)said drill head assembly will be locked to said drive assembly forrotation therewith when said inner drill member is forced toward saiddrive adaptor far enough to place said coupling member in said firstlimit position; and said bearing means being disposed between said drivemember and said coupling member so as to facilitate rotation of saiddrive member relative to said coupling member.
 15. An improved drillingimplement according to claim 14 further including sealing means betweensaid drive member and said coupling member for preventing introductionof foreign matter into said bearing means.
 16. An improved drillingimplement according to claim 14 further including a mandrel connected tosaid drive adaptor, and further wherein said biasing means is a coiledspring having one end mounted on said mandrel.
 17. A drilling implementfor drilling holes in bone structures comprising a drill head assemblyand a drill head drive assembly;said drill head assembly comprising ahollow outer drill member and an inner drill member coaxially disposedwithin said outer drill member; said outer drill member comprising anouter end having a plurality of cutting flutes and an inner end having aplurality of circumferentially-spaced lips that extend in a plane atright angles to the longitudinal axis of said outer drill, each of saidlips having a drive surface extending in a plane that intersects theplane of said lips; said inner drill member comprising an outer endhaving a plurality of cutting flutes and an inner end having (1) an endsurface, (2) a plurality of mutually-spaced, circumferentially-extendinglugs projecting from said end surface parallel to the longitudinal axisof said inner drill member, each of said lugs having a cam surfaceextending circumferentially and at an angle to the longitudinal axis ofsaid inner drill member and disposed to engage one of said drivesurfaces and thereby drive said outer drill member when said inner drillmember is rotated in a first direction, and (3) a threaded center hole;said flutes of said inner drill member having front end surfacesterminating in front cutting edges and said flutes of said outer drillmember having front end surfaces terminating in front cutting edges,with said front end surfaces of said outer drill member being disposedat a shallower angle to the longitudinal axis of said drilling implementthan said front end surface of said inner drill member, whereby saidouter drill member will encounter greater resistance to cutting thansaid inner drill member; said drill head drive assembly comprising adrive member, a drill head/drill drive coupling member, a drive adaptor,and biasing means; said drive member comprising an end surface extendingtransversely of its axis and interrupted by a plurality of recesses forreceiving said plurality of lugs, each of said recesses being defined inpart by side surfaces extending in planes extending parallel to thecenter axis of said drive member; said coupling member being screwedinto said threaded center hole so that it and said inner drill memberwill rotate as a unit when said inner drill member is rotated in a saidfirst direction; said drive adaptor being connected to said drive memberso that said drive adaptor and said drive member will rotate as a unitwhen said drive adaptor is rotated in said first direction; and saidbiasing means being disposed so as to bias said coupling member and saidinner drill member axially away from said drive adaptor; said drill headassembly and said drive assembly being arranged so that when said driveadaptor is rotated in said first direction, (a) so long as said innerdrill member encounters a predetermined resistance to penetration, saidlugs will remain engaged with said side surfaces and said inner drillmember will cause said outer drill member to rotate with it as a unitwith said drive adaptor, and (b) when said inner drill member no longerencounters said predetermined resistance to penetration, said camsurfaces will coact with said drive surfaces to force said inner drillmember forward relative to said outer drill member and said drill headdrive assembly sufficiently for said lugs and said side surface to bedisengaged, whereby said inner and outer drill members will sliprelative to-said drive member as said drive adaptor is rotated in saidfirst direction.
 18. A drilling implement for drilling holes in bonestructures comprising a drill head assembly and a drill head driveassembly;said drill head assembly comprising a hollow outer drill memberand an inner drill member coaxially disposed within said outer drillmember; said outer drill member comprising an outer end having aplurality of cutting flutes and an inner end having threecircumferentially-spaced lips that extend in a plane at right angles tothe longitudinal axis of said outer drill, each of said lips having adrive surface extending in a plane that intersects the plane of saidlips and said longitudinal axis; said inner drill member comprising anouter end having a plurality of cutting flutes and an inner end having(1) an end surface, (2) three mutually-spaced,circumferentially-extending lugs projecting from said end surfaceparallel to the longitudinal axis of said inner drill member, each ofsaid lugs having a cam surface extending circumferentially and at anangle to the longitudinal axis of said inner drill member and disposedto engage one of said drive surfaces and thereby drive said outer drillmember when said inner drill member is rotated in a first direction, and(3) a threaded center hole; said flutes of said inner drill memberhaving front end surfaces terminating in front cutting edges and saidflutes of said outer drill member having front end surfaces terminatingin front cutting edges, with said front end surfaces of said outer drillmember being disposed at a shallower angle to the longitudinal axis ofsaid drilling implement than said front end surface of said inner drillmember, whereby said outer drill member will encounter greaterresistance to cutting than said inner drill member;said inner drillmember also having a pyramidal center point characterized by threeconverging sides projecting forwardly from said front end surfaces; saiddrill head drive assembly comprising a drive member, a drill head/drilldrive coupling member, a drive adaptor, and biasing means; said drivemember comprising an end surface extending transversely of its axis andinterrupted by three recesses for receiving said three lugs, each ofsaid recesses being defined in part by side surfaces extending in planesextending parallel to the center axis of said drive member; saidcoupling member being screwed into said threaded center hole so that itand said inner drill member will rotate as a unit when said inner drillmember is rotated in a said first direction; said drive adaptor beingconnected to said drive member so that said drive adaptor and said drivemember will rotate as a unit when said drive adaptor is rotated in saidfirst direction; said biasing means being and disposed so as to biassaid coupling member and said inner drill member axially away from saiddrive adaptor; said drill head assembly and said drive assembly beingarranged so that when said drive adaptor is rotated in said firstdirection, (a) so long as said inner drill member encounters apredetermined resistance to penetration, said lugs will remain engagedwith said side surfaces and said inner drill member will cause saidouter drill member to rotate with it as a unit with said drive adaptor,and (b) when said inner drill member no longer encounters saidpredetermined resistance to penetration, said cam surfaces will coactwith said drive surfaces to force said inner drill member forwardrelative to said outer drill member and said drill head drive assemblysufficiently for said lugs and said side surfaces to be disengaged,whereby said inner and outer drill members will slip relative to saiddrive member as said drive adaptor is rotated in said first direction.